1. Field of the Invention
The present invention relates to an optical switch which changes over the transmission paths of an optical signal by a refractive index change, and more particularly to an optical switch which enables to reduce crosstalk.
2. Description of the Related Art
LAN (Local Area Network), WAN (Wide Area Network), etc. which are present-day communication networks, are based on communication schemes in which information items are usually transmitted with electric signals.
A communication method which transmits information with an optical signal, is employed in only a trunk network and some other networks in which large quantities of data are transmitted. Besides, these networks perform “point to point” communications, and it is the actual situation that they have not yet progressed to a communication network which can be termed a “photonic network”.
In order to realize such a “photonic network”, there are necessitated an “optical router”, an “optical switching hub”, etc. which have functions similar to those of devices such as a router and a switching hub for changing over the transmission destinations of electric signals.
Besides, such a device requires an optical switch for changing over transmission paths at high speed. Known as the optical switch is one which employs a ferroelectric such as lithium niobate or PLZT (Lead Lanthanum Zirconate Titanate), or one in which optical waveguides are formed in a semiconductor, and carriers are injected into the semiconductor so as to change a refractive index and to change over the transmission paths of an optical signal.
Further, there is recently known an optical switch in which heat is generated by a heater integrated on a plane-glass optical waveguide, and the refractive index of a part formed with the heater is changed, thereby to perform a switching operation.
The following documents are referred to as a related-art optical switch in which optical waveguides are formed in a semiconductor, and carriers are injected into the semiconductor so as to change a refractive index and to change over the transmission paths of an optical signal.
Document 1: JP-A-06-059294
Document 2: JP-A-06-130236
Document 3: JP-A-06-194696
Document 4: JP-A-2004-020909
Document 5: JP-A-2004-264631
Document 6: “2×2 Optical Waveguide Switch with Bow-Tie Electrode Based on Carrier-Injection Total Internal Reflection in SiGe Alloy”, Baojun Li and Soo-Jin Chua, p206-p208, IEEE PHOTONIC TECHNOLOGY LETTERS, VOL. 13, NO. 3, MARCH 2001
FIGS. 3 and 4 are a plan view and a sectional view showing an example of the related-art optical switch stated in document 6, respectively. In FIG. 3, numeral 1 designates an optical waveguide layer having “X-shaped” optical waveguides “WG01” and “WG02”, and numerals 2 and 3 designate a pair of electrodes for injecting carriers.
Referring to FIG. 3, the “X-shaped” optical waveguides are formed on the optical waveguide layer 1, and the oblong electrode 2 is formed at the crossing part of the “X-shaped” optical waveguides as shown at “CP01” in FIG. 3. Besides, the oblong electrode 3 is formed near the crossing part of the “X-shaped” optical waveguides and in parallel with the electrode 2.
On the other hand, FIG. 4 is the sectional view taken along line “A-A′” in FIG. 3. Referring to FIG. 4, numeral 1 and signs “WG01” and “WG02” are the same as in FIG. 3, respectively. Further, numeral 4 designates a clad layer, and numeral 5 a substrate.
The clad layer 4 and the optical waveguide layer 1 are successively formed on the substrate 5, and the “X-shaped” optical waveguides as shown at “WG01” and “WG02” in FIG. 4 are formed in the optical waveguide layer 1. Besides, the optical waveguides formed in the optical waveguide layer 1 are ones of ridge type, and an optical signal is distributively propagated as shown at “PS01” in FIG. 4 by way of example.
Here, the operation of the related-art example shown in FIG. 3 will be described with reference to FIG. 4. In a case where the optical switch is “OFF”, any current is not fed to the electrodes 2 and 3.
Therefore, the refractive index of the crossing part of the “X-shaped” optical waveguides as shown at “CP01” in FIG. 3 does not change. Consequently, by way of example, the optical signal inputted from an input or entrance end shown at “PI01” in FIG. 3 proceeds rectilinearly through the crossing part and is outputted from an output or exit end shown at “PO01” in FIG. 3.
In contrast, in a case where the optical switch is “ON”, electrons are injected from the electrode 2, and holes are injected from the electrode 3, so that the carriers (electrons, holes) are injected into the crossing part.
Owing to a plasma effect, therefore, the refractive index of the crossing part of the “X-shaped” optical waveguides as shown at “CP01” in FIG. 3 changes so as to lower. Consequently, by way of example, the optical signal inputted from the input end shown at “PI01” in FIG. 3 is totally reflected at the low index part generated at the crossing part shown at “CP01” in FIG. 3 and is outputted from an output or exit end shown at “PO02” in FIG. 3.
As a result, the position from which the optical signal is outputted can be controlled, in other words, the propagation path of the optical signal can be changed over, in such a way that the refractive index of the crossing part of the “X-shaped” optical waveguides is controlled by feeding currents to the electrodes and injecting the carriers (holes, electrons) into the crossing part.
With the related-art example shown in FIGS. 3 and 4, the path of the optical signal was examined by simulations, etc. Then, although the optical waveguides through which the optical signal is propagated are changed over by the “ON/OFF” operation of the optical switch, there has been the problem that part of the optical signal leaks out to a part outside the optical waveguides irrespective of the “ON” or “OFF” state of the optical switch.
As the cause of such a problem, it is conjectured that, since the shapes of at least two optical waveguides change conspicuously at a part where the waveguides cross, the waveguiding mode of light will change to incur reflection or scatter.
That is, in an optical switch in which the plurality of optical waveguides cross, the leakage of the light attributed to the reflection or scatter exists more or less.
In the configuration of the optical switch as shown in FIG. 3, such leakage of the light is little influential for the reason that the light having leaked out to the part outside the optical waveguides is difficult of arriving at the output ends shown at “PO01” and “PO02” in FIG. 3.
However, in a case where the optical switches as shown in FIG. 3 are integrated as shown in FIG. 5, the light having leaked out to the part outside the optical waveguides will flow into (combine with) another optical waveguide again at a high possibility.
FIG. 5 is a plan view of the optical switch in which the optical switches as shown in FIG. 3 are integrated. Referring to FIG. 5, numeral 6 designates an optical waveguide layer having “X-shaped” optical waveguides “WG11” and “WG12”, and also having “y-shaped” optical waveguides which branch at different angles from the intermediate parts of the “X-shaped” optical waveguides on the output or exit end sides thereof, respectively.
Regarding the optical switch in FIG. 5, electrode pairs for injecting carriers are respectively required at the crossing part of the optical waveguides as shown at “CP11” in the figure, and the branch portions of the optical waveguides on the output end sides thereof as shown at “BP11” and “BP12” in the figure. In FIG. 5, however, the electrode pairs are omitted from illustration.
Here, the operation of the related-art example shown in FIG. 5 will be briefly described. By way of example, on condition that the crossing part shown at “CP11” in FIG. 5 has its refractive index lowered by the injection of the carriers (in an “ON” state), an optical signal inputted from an input or entrance end shown at “PI11” in the figure is reflected toward the branch portion shown at “BP11” in the figure. Subsequently, on condition that the branch portion shown at “BP11” in FIG. 5 has its refractive index lowered by the injection of the carriers (in an “ON” state), the optical signal is further reflected by the branch portion shown at “BP11” in the figure and is outputted from an output or exit end shown at “PO12” in the figure.
Also, by way of example, on condition that the carriers are not injected into the crossing part shown at “CP11” in FIG. 5 (in an “OFF” state), the optical signal inputted from the input end shown at “PI11” in the figure proceeds rectilinearly toward the branch portion shown at “BP12” in the figure. Subsequently, on condition that the carriers are not injected into the branch portion shown at “BP12” in FIG. 5 (in an “OFF” state), the optical signal further proceeds rectilinearly and is outputted from an output end shown at “PO14” in the figure.
That is, the optical signal inputted from the input end shown at “PI11” in FIG. 5 can be outputted from any of output ends shown at “PO11”, “PO12”, “PO13” and “PO14” in the figure, by controlling the injections of the carriers into the crossing part shown at “CP11” in the figure and the branch portions shown at “BP11” and “BP12” in the figure.
Here, light which has leaked out to a part outside the X-shaped optical waveguides at the crossing part shown at “CP11” in FIG. 5, as shown at “LK11” in the figure, will flow into (combine with) the optical waveguide as shown at “WG13” or “WG14” in the figure, at a high possibility.
By way of example, even in a case where the optical switch is controlled so as to output the optical signal from the output end shown at “PO14” in FIG. 5, by switching, an optical signal is outputted also from the output end shown at “PO12” or “PO13” in the figure, in other words, crosstalk occurs.